With recent developments of industry in semiconductors and optoelectronics, use of highly toxic hydrides such as arsine, phosphine, diborane and hydrogen selenide has markedly increased. These toxic components are essentially required as raw materials or doping gases in the production of silicon semi-conductors, compound semi-conductors or optical fibers.
Exhaust gases discharged from the process for the production of semi-conductors or optical fibers generally contain unreacted toxic components and, in view of their high toxicity to living organisms, these toxic components must be removed from the exhaust gases before these gases are discharged to open atmosphere in order to prevent environmental pollution.
Known methods for removing such toxic components from the exhaust gas include a wet process which comprises absorbing and decomposing the toxic components in a scrubber, and a dry process which comprises passing the exhaust gas through a column packed with a cleaning agent such as adsorbents or oxidizing agents.
However, the conventional wet process generally has problems such as corrosion of the apparatus due to the use of an absorbing liquid and post-treatment of the absorbing liquid, and thus the wet process is expensive for maintenance of the apparatus.
On the other hand, the conventional process using a cleaning agent comprises oxidative removal of phosphine or arsine using an adsorbing agent, for example, nitrates such as silver nitrate supported on a porous carrier, or a porous carrier impregnated with a metal chloride such as ferric chloride, as disclosed in Japanese patent application (OPI) No. 89837/81 (the term "OPI" as used herein means a published unexamined patent application). Although this conventional process has solved the problems associated with the above-described wet process, it still has problems such that the exhaust gas discharged from the chemical vapor deposition (CVD), ion implantation or doping process must be preliminarily subjected to wet-treatment thereby requiring complicated apparatus.
A further conventional process comprises treating arsine and phosphine using three different absorbents, i.e., inorganic silicates impregnated separately with (1) an aqueous solution of an alkali, (2) an aqueous solution of an oxidizing agent, and (3) an aqueous solution of alkali and an oxidizing agent, as disclosed in Japanese patent publication No. 49822/84. However, this conventional process also includes treatments under wet conditions and, therefore, has disadvantages as described above for the wet process.
A typical dry process for removing arsine used as chemical weapon was to use a gas mask filled with activated carbon. By taking advantage of adsorption ability of activated carbon, various attempts have been made to improve the performace of activated carbon by incorporating and absorbing various materials into activated carbon. For example, Japanese patent application (OPI) No. 160,535/84 discloses arsine-adsorbing agents comprising activated carbon as a carrier having incorporated therein a copper compound, an alkali metal compound, an alkaline earth metal compound, and at least one compound of Al, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Cd and Pb. Although this adsorbing agent is advantageous in that it can be used in completely dry process, the adsorbing agent of this type is not satisfactory in its relatively weak arsine removal property. Further, since the above adsorbing agent generates heat upon contact with air after adsorption of arsine and has a risk in that the activated carbon may ignite depending upon conditions, its use in industry is severely limited.
As a result of extensive studies for eliminating disadvantages in the conventional process, the present inventors found that the above-described toxic components can be effectively removed by contacting an exhaust gas containing these toxic components with a cleaning agent having a composition consisting essentially of (1) cupric oxide and (2) at least one metal oxide selected from the group consisting of silicon oxide, aluminum oxide and zinc oxide.
Thus, the present invention relates to a method for cleaning an exhaust gas containing at least one toxic component selected from the group consisting of arsine, phosphine, diborane and hydrogen selenide, which comprises contacting the toxic component with a molded cleaning agent having a composition consisting essentially of (1) cupric oxide and (2) at least one metal oxide selected from the group consisting of silicon oxide, aluminum oxide and zinc oxide and having a density of from about 1.5 to about 3.5 g/ml, said composition having a metal atomic ratio M/(M+Cu) in the range of from about 0.02 to about 0.7 wherein Cu represents a number of gram atom of copper and M represents a total number of gram atom of silicon, aluminum and/or zinc, to remove the toxic component from the exhaust gas.